For producing a casting such as of aluminum according to a casting process, molten aluminum is poured into a casting die. Since the molten aluminum is of a high temperature, the casting die is generally made of an SKD61 material (Japanese Industrial Standard for representing an alloy tool steel) which is of excellent strength at high temperatures.
If a casting die suffers heat cracking, then it is difficult to produce aluminum castings of desired dimensional accuracy from the casting die. Stated otherwise, the yield of aluminum castings from the casting die becomes low. Accordingly, when heat cracking occurs even in a portion of a casting die, the casting die needs to be replaced with a new one even though the remaining portion of the casting die is free of any heat cracks. Since casting dies are generally expensive, however, the manufactured aluminum castings become highly costly if they are produced from frequently replaced casting dies.
Heat cracking occurs in a casting die when the temperature of the casting die changes abruptly by contact with a high-temperature molten metal that is poured into the casting die, i.e., when the casting die is subjected to a thermal shock. Consequently, casting dies are required to be resistant to thermal shocks.
To make casting dies resistant to thermal shocks, the casting dies are usually processed by surface treatments. Specifically, casting dies are coated with a ceramic layer such as of TiC, TiN, or the like by a salt bath process, a nitriding process such as a gas nitriding process or an ionitriding process, a physical vapor deposition (PVD) process, or a chemical vapor deposition (CVD) process, or coated with a layer comprising a mixture of iron sulfide and iron nitride by a sulphonitriding process, or coated with an oxide layer of iron oxide by an oxidizing process.
However, it is difficult to greatly increase the service life of casting dies even if they are processed by the above surface treatments. Specifically, portions of casting dies which are subject to intensive heat shocks, e.g., mold recesses that are present in the vicinity of a gate for receiving an introduced molten metal, of all mold recesses having a horizontal side wall which extends horizontally and a vertical bottom wall which extends substantially vertically, are susceptible to heat cracks even though they are coated with layers applied by the above surface treatments.
It has been proposed to apply carburizing to mold portions that are to be resistant to thermal shocks, as disclosed in Japanese Laid-Open Patent Publication No. 2002-121643. However, the proposed carburizing process fails to greatly increase the thermal shock resistance of carburized mold portions, and to greatly lower the cost of castings that are manufactured by the carburized casting dies.